Most gypsum plasters (anhydrite screed included in this) will tolerate a small quantity of clean ordinary portland cement. Different manufacturers argue on the percentage but vary between 3 and 5%. This can actually be beneficial in terms of surface hardness as the cement hydrates relatively quickly. It does have the disadvantage of reducing the flexural strength of the material. In fact most Hemi Hydrate Screed materials have around 4% cement in them.
If however, as is more likely these days, he has used a modern bagged cement, many of which contain plasticisers (e.g. Multicem and Mastercrete) these can severely retard the setting of the gypsum as they impede the chrystalisation process.
I occasionally see issues where a concrete mixer has failed to wash out thoroughly and the anhydrite gets contaminated with polycarboxylate plasticiser and it can take several days for the screed to go hard. Unless there has been a significant level of plas added though it will go hard it could just take several days. The dampness in the wall will not have helped and ought to have been dealt with first as it could affect the long term bond of the plaster to the wall itself.
In fact when I have wanted to slow down the setting time of out of date plaster I have added a couple of cap fuls of detol (acid retarder) and have been able to work with it. Mind you the smell was horrid.
If it was a really modern cement which is blended with GGBS (a cement replacement) then you might as well take it off the wall now. This latter is unlikely. I am not sure phoenix (trade name for Blue Circle) is available in bagged form. GGBS and Gypsum are very poor bedfellows and the latter leads to poly sulphide attack in the Gypsum core causing aggresively expansive degeneration of the gypsum.
The shelf life of cement is to do with the removal a few years ago of Chromium 6 from the cement. This acts as a stabilising agent to prevent the cement hydrating whilst in storage. Hexavalent Chromium is probably the best material for this purpose but unfortunately it is also responsible for the the dermatitis people got when using cement. It was replaced predominantly with Ferrous Oxide which is less effective so manufacturers now put a 3 to 6 month shelf life on cement but at least it does not give you cancer any more.
I think realistically it is most likey the result of the addition of plasticised cement and whilst it might take longer than it ought to go hard there is little need to worry.
